A method of determining the temperature of a sample carrier in a charged particle-optical apparatus is known from the Gatan 914 High Tilt Liquid Nitrogen Cryo Transfer Tomography Holder, commercially available from Gatan Inc., Pleasanton, Calif., USA. This holder is used for cryo-tomography in a Transmission Electron Microscope (TEM) or Scanning Transmission Electron Microscope (STEM), and is equipped to hold sample carriers (“grids”) which in turn carry a sample. According to the manufacturer the temperature of the holder is monitored by a calibrated silicon diode that provides a sensitive, linear temperature response. This diode is located in the sample holder.
A disadvantage of said measurement method is that the measurement measures the temperature of a part removed from the sample itself: the temperature of a part of the sample holder is measured, in which the sample carrier is clamped. The sample carrier is typically a perforated copper foil with a diameter of 3 mm diameter and a thickness of 20 μm or less. The temperature of the region of interest on the thin foil can thus differ from the temperature measured by the measuring diode.
Another disadvantage of the measurement is that the measurement comprises feeding a current through the diode and measuring the voltage over the diode. The measurement thus raises the temperature of the diode by Ohmic heating, which is, especially at cryogenic temperatures, unwanted.
Yet another disadvantage of said method is that the use of the diode implies that thin wires must be introduced in the sample holder, a side-entry type holder, which results in a complex product with high costs.
Another known method is the measurement of the temperature using a pyrometer. This method of measuring temperatures is known per se.
A disadvantage of this method is that, as known to the person skilled in the art, this method is well suited for measuring temperatures above, for example, 500° C., but is not suited for measuring at room temperatures, and even less at cryogenic temperatures.
Another disadvantage is that an optical transparent window with a view to the sample must be available to enable the pyrometer to view the sample. Especially in a STEM or a TEM, where the sample is placed between the pole pieces of the so-called objective lens, this volume between the pole pieces is at a premium, as is access to said volume.
It is mentioned that sample inspection at cryogenic temperatures is, as known to the skilled artisan, important for inspecting biological samples, for example to reduce damage to the samples and to eliminate the otherwise needed embedding in resin.
It is noted that a method is known from U.S. Pat. No. 7,331,709 disclosing a carbon nanotube with a diameter of between 40 and 150 nm and a length of between 1 and 10 μm. The carbon nanotube is closed at one end and encloses a small droplet or column of gallium. To measure a temperature in the range of 50° C. to 500° C. the carbon nanotube together with the column of gallium is brought to the specimen to be measured, the specimen located in atmosphere. As a result, a cap of oxidized gallium is formed in the carbon nanotube, that stays in position after cooling down. The position of the cap can be determined by a TEM (it is noted that a carbon nanotube is transparent to the electrons). The position is linked to the temperature at which the cap is formed by the thermal expansion coefficient of gallium, and thus the temperature difference between the temperature at which the TEM measures the position and the temperature at which the cap is formed (or at least pushed in position) is determined.
As this method is based on oxidizing gallium in an atmospheric environment, it is not suited to be used in the vacuum of a particle-optical apparatus.